Normally, a brake pressure (primary pressure) corresponding to the degree of depression of a brake pedal being operated by a driver is applied to a wheel brake from a brake master cylinder. A moving speed (reference speed) of a vehicle is estimated from the rotational speed of a plurality of wheels. The slip rate of wheels or the coefficient of friction, .mu., of a road surface is calculated or estimated from the reference speed and the rotational speed of wheels, and the wheel brake pressure is decompressed so as to avoid a complete interruption of rotation of the wheels (a wheel lock) even through the vehicle continues its movement. Subsequently the brake pressure is compressed or intensified so that the stroke through which the braking is applied be minimized, and such decompression and compression are repeated as required. Such operation is commonly referred to as an antiskid control. Such an antiskid control is enabled by the provision of a source of pressure comprising an intensifying/reducing valve which decompresses or compresses a wheel brake pressure, a fluid pump for supplying a pressure (secondary pressure) higher than the primary pressure, and an electric motor which drives the pump. The antiskid control is executed by an electronic controller, and when it determines that there is a need to change a wheel brake pressure (automatic intervention), it causes the pressure source to supply the secondary pressure to the valve, which then selectively switches the wheel brake pressure between a low pressure (drain pressure) and the secondary pressure. When the low pressure is supplied, the wheel brake pressure is reduced while when the secondary pressure is supplied, the wheel brake pressure rises. One form of such antiskid control is disclosed in Japanese Laid-Open Patent Application No. 38,175/1990.
in addition to controlling the wheel brake pressure from the viewpoint of the slip rate of the wheels and the distance across which they are braked as a vehicle is being braked, another form of controlling the wheel brake pressure is recently proposed in which a distribution of braking efforts applied to the front and rear, and left and right wheel brakes, which secures a directional stability of a vehicle being braked in accordance with the driving and/or running condition of the vehicle as well as a load distribution on the vehicle, is calculated by an electronic controller. The intensifying/reducing valve is used to regulate the wheel brake pressure to achieve such distribution. The present inventors have previously proposed such system in Japanese Laid-Open Patent Applications No. 85,327/1993, No. 85,340/1993 and No. 85,336/1993. A wheel brake pressure system for such antiskid control and the braking efforts distribution control in which the secondary pressure applied to the intensifying/reducing valve is chosen to be a hydrobooster pressure, which is on the order of 20%, for example, higher than the primary pressure, is disclosed in Japanese Laid-Open Patent Application No. 144,179/1994.
In all these applications cited above, the depression of brake pedal by a driver is a prerequisite to the initiation of the antiskid control as a matter of course, and also to initiate the braking efforts distribution control. In the antiskid control, if a braking action applied to the wheels as a result of the depression of a brake pedal results in an overbraking, the wheel brake pressure is once decompressed, and is then intensified so as to achieve a desirable slip rate of the wheels and to minimize the distance across which the wheels are being braked. In addition, depending on a decision rendered by a driver, the brake pressure is also operated through the brake pedal, and accordingly, it is reasonable to employ the hydrobooster pressure which is responsive to the depression or release of the brake pedal for the compression during the antiskid control. However, in controlling the braking efforts distribution, the purpose is to regulate the brake pressures applied to the individual wheels so as to realize a distribution of individual wheel brake pressures (applied to front and rear, and left and right wheel brakes) which is advantageous in assuring a directional stability and a steerability of a vehicle so as to accommodate for the driving/running condition of the vehicle and a load distribution on the vehicle. Accordingly, the use of the hydrobooster pressure may fail to realize a required distribution of the braking efforts. For example, if the depression of the brake pedal is small, the resulting hydrobooster pressure is low, while a higher brake pressure may be required to achieve the distribution of the braking efforts.
During a rapid start without depression of a brake pedal or during a quick turn, a temporary shift may occur in a distribution of load on the vehicle or the coefficient of friction with the road surface may vary from wheel to wheel, both of which may degrade the directional stability or steerability of the vehicle. In such instance, it is preferable to have a control over the distribution of the braking efforts, but since the depression of the brake pedal is lacking, the wheel brake pressure cannot be increased by the hydrobooster pressure. During the control of the distribution of the braking efforts, it is preferred that the brake pressure of each of the four wheels can be individually controlled, even though the directional stability and the steerability can be maintained by the control over the distribution of the braking efforts, applied to a specific one or more of the wheel brake pressures without individually controlling the brake pressure of all the wheels depending on the type of the vehicle (the location of an engine, the location of a driver's seat, the distribution of seats, front drive/rear drive/all wheel drive, front wheel steering/rear wheel steering/front and rear wheel steering, the presence or absence of luggage carrier).